If Scott comes back, then "death" is meaningless in the X-franchise and the movies becomes less real.

Killing Scott off left the series in a real hole. Marsden was someone who could have been resigned for several more movies for considerably less than Hugh Jackman or Halle Berry.

However, Scott without Jean leaves the series also with serious problems.

Scott became one of the most hated characters in all of comics for 1) abandoning his wife and son when Jean returned and 2) cheating on Jean with Emma and then starting a relationship with Emma days after his wife's death.

So, if they introduce Madelyne Pryor or Emma in any sequels and hook them up with Scott, they either can't bring back Jean or they have to make them evil to make Scott not look like the cheating scum he is.

So without Scott and Jean, it's a loose-loose situation and the X-franchise gets locked in a corner.